The present invention relates to equipment for heating polar polymers to the temperature at which they become plastic, by making use of the interaction between the electric dipole moment of the polymers and the electromagnetic field associated with a microwave.
Microwaves, that is to say electromagnetic waves of which the wavelengths are between those of radio waves and those of infra-red waves have already been used, with a certain amount of success, to heat various materials.
Thus, microwave ovens have already been proposed especially for reheating and cooking food. Since the wavelength of these microwaves is of the order of a centimeter, these radiations can be guided through tubular waveguides and can form stationary waves in closed or semi-closed cavities such as ovens.
More recently, tunnel ovens have also been developed and these are successfully used for drying or gelling aqueous dispersions or the like of a polar plastic material deposited on temporary or final supports.
The use of these waves as a means for heating polar materials in fact possesses some advantages compared with the radiations of less high frequencies widely used hitherto.
Thus, the absorption of energy by the treated material is higher, since this absorption is a function of the reciprocal wavelength. Moreover the electrodes need not be located in the heating zone.
Microwaves thus form a very valuable means of heating and their use in customary pieces of equipment for processing polymers, such as, for example, extruders and other plasticising or gelling devices, is highly desirable.
However, the use of microwaves as a means for heating has never been exploited hitherto in conventional pieces of equipment, probably due to the fact that specialists have considered that the presence of screws in these pieces of equipment excluded any possibility of making use of these radiations.
Moreover, microwaves have never been exploited either for processing polymers in equipment specially designed for this purpose. In fact, a very important phenomenon must be taken into consideration when the heating of certain polar polymers, such, as, for example, vinyl polymers, by means of electromagnetic radiation is considered. It has been found, that in many cases, the capacity of these materials for absorbing energy increases with their temperature. The result of this is that if, for any reason whatsoever, the initial temperature of these products is heterogeneous, the absorption of energy is most intense at the hottest points. This results in preferential heating of the hot points and, taking into account the low thermal conductivity of these materials, rapidly leads to degradation, especially when the material is heat-sensitive.
Moreover, since it is not possible, when a microwave is travelling through a waveguide, to achieve a constant energy distribution at every point throughout the crosssection of the waveguide, if the material to be heated travels at a constant speed through the waveguide, this inevitably leads to preferential localised overheating effects which are amplified and present the danger of degrading the treated material.
Thus it is generally found that, when a polar polymer is passed at a constant linear speed through a rectilinear waveguide of circular cross-section supplied by a microwave generator and having given dimensions so that only the mode of longest wavelength can propagate, the central part of the flow has a tendency to become hotter than the periphery and this difference increases very rapidly as soon as it is triggered.
These phenomena are obviously extremely objectionable when the use of microwaves is considered for heating a polar polymer by making it travel through a waveguide.